1. Field of the Invention:
The present invention relates to a picking device for a fluid jet loom and, more particularly, to a weft yarn measuring method which enables the accurate measurement of a weft yarn of a necessary length.
2. Description of the Prior Art:
A drum type weft yarn measuring and storing device measures a weft yarn of a necessary length by winding the weft yarn on a stationary storage drum with a rotary yarn guide while a retaining pin retains the weft yarn on the storage drum. At a picking phase, the retaining pin is retracted from the circumference of the storage drum to release the weft yarn wound on the storage drum, and then the weft yarn is picked into a shed by a jet of fluid jetted by a picking nozzle.
In measuring the weft yarn, the length of yarn unwound on the storage drum is determined by the number of loops of the weft yarn on the storage drum. Japanese Laid-Open Patent Publication No. 57-29640 discloses an invention for unwinding the weft yarn of a length necessary for one picking cycle, in which the number of loops of the weft yarn unwound from a storage drum is counted by a photoelectric sensor disposed near the circumference of the storage drum. It is possible that such a photoelectric counting means fail to detect the quick passage of a thin weft yarn across a counting position and that the photoelectric counting means fails to count the number of loops accurately because fly and dust are liable to be deposited on the light receiving surface of the photoelectric counting means.
On the other hand, a device disclosed in Japanese Laid-Open Utility Model Publication No. 61-164288 releases a plurality of loops of a weft yarn necessary for one picking cycle from a storage drum by controlling the duration of retraction of a retaining pin. This device, however, is unable to release the predetermined plurality of loops of the weft yarn accurately, because the running mode of the picked weft yarn with respect to a time axis is variable due to variations in the physical properties of the weft yarn and variations in the pressure of the picking fluid. That is, although the retaining pin is retracted for a predetermined period of time, the running speed of the picked weft yarn is caused to vary by the variations of the physical properties of the weft yarn and the variations of the pressure of the picking fluid and, consequently, the number of loops of the weft yarn unwound in the predetermined period of time during which the retaining pin is held at the retracted position varies. The greater the number of loops of the weft yarn necessary for one picking cycle, the greater is error in the measurement.